We’ve always (well, since the fall of ’08 anyway) had an RSS feed for the comments on any particular post, but I never got around to implementing email notification for comment replies, support for which is built into the WordPress core but which was never set up in this ancient (and admittedly much-rejiggered) theme.
It’s now in place via a check-box below the Submit button accompanying the comment box. I suspect it works better if you tick the box before you save your comment.
Thanks to Tatyana, who wound up caught up in the testing routine. (Much to my complete lack of surprise, I did not get things to work on the first try.) If you’re keeping score, we’re using Subscribe to Comments by Mark Jaquith, WordPress Director of Whitespace; I bought him a drink, or something.